Leptospirosis is a neglected emerging zoonotic disease with worldwide distribution that affects virtually all vertebrates. It is a burden on society due to elevated morbidity in humans (~1 million cases a year, 5 to 10% mortality rate) and extensive loss of animals of agricultural interest. Traditionally, leptospirosis is more common in tropical regions. However, an outbreak was reported in New York City recently. There is no vaccine approved for human use in the US and antibiotic treatment is only effective if used early in the course of infection. Critical barriers in the development of effective countermeasures against leptospirosis are: 1) absence of an inbred immunocompetent mouse model amenable to in-depth characterization of host factors underlying virulence, disease pathology and vaccine efficacy mechanisms; and 2) lack of non-toxic recombinant based shedding-blocking vaccines. The ultimate goal of this project is to develop immunocompetent mouse models of persistent leptospirosis, suitable for consistent measurement of Leptospira shedding in urine. Another goal is to test new, bi-functional recombinant based vaccine candidates, as well as established bacterins, to evaluate efficacy of new vaccine candidates and validate the mouse model. Our central hypothesis is that we can use TLR4 humanized transgenic mice as immunocompetent models of leptospirosis. The short-term impact of this 2-year proposal relates to development of mouse models that mimic sublethal and chronic leptospirosis that can be used to test shedding-blocking vaccines as well as adjuvants that signal through TLR4. The long-term impact of this proposal is conceptual and relates to the study of persistence of a spirochetal pathogen in an immunocompetent mouse model.